Okay so it's supposed to be possible to put your sculpture into your inventory like food and stuff, but it's un-dragable and there's no option he can click that results in the object moving to his inventory.

He's supposed to be able to sell his sculptures to the local consignment store at that, which is only possible if he drags it in his inventory, and now he has this mission thing where he has to deliver a masterpeace to some public place, but to do this it needs to be in his inventory.

Is this yet again a bug or am I doing something wrong here?

I haven't done that career yet but are you sure you aren't supposed to put it in the family inventory in buy mode?

hmmm.. nope xD I'm not sure, it doesn't really explain anywhere :( I'll give that a try thanks ^-^